1. Field of the Invention
This invention relates to an angular rate sensor suitable for sensing motion about at least one axis.
2. Disscussion of Prior Art
Conventional angular rate sensors such as vibrating structure gyroscopes may be constructed using a variety of different vibrating structures. These include beams, tuning forks, cylinders, hemispherical shells and rings. A common feature in all of these structures is that they maintain a resonant carrier mode oscillation. This provides the linear momentum which produces a Coriolis force when the gyroscope is rotated around the appropriate axis. This induced force will produce an oscillatory motion, at the frequency of the carrier mode, along an axis perpendicular to that of the applied rotation and linear momentum. The amplitude of the oscillatory motion will be directly proportional to the applied rate.
The sensitivity of such vibrating structure gyroscopes may be enhanced by designing the vibrating structure, that is, the resonator or vibrating element, such that the Coriolis force directly excites a natural vibration mode of the structure. If the frequency of this response mode exactly matches that of the carrier frequency then the amplitude of the response mode motion will be amplified by the mechanical quality factor, Q, of the structure. Achieving this matching of carrier and response mode frequencies inevitably places tight constraints on the construction tolerances. In practice, it is usually necessary to fine tune the balance of the resonator by adding or removing material at appropriate points around the resonator. This locally adjusts the mass or stiffness parameters thus differentially shifting the mode frequencies.
There are many examples of conventional vibrating structure gyroscopes fabricated using traditional machining techniques. These include a ceramic cylinder vibrating structure gyroscope and Hemispherical Resonator Gyroscope. Recently, advances in micromachining techniques have provided the capability for manufacturing vibrating structures of significantly reduced size and mass and at substantially lower cost. This has in turn opened up new market opportunities for vibrating structure gyroscopes in areas such as car navigation and chassis control systems.
The requirement to balance accurately the carrier and response mode frequencies is equally applicable to micromachined vibrating structure gyroscopes. Lithographic techniques conventionally employed in micromachining fabrication of silicon vibrating structures are capable of achieving extremely high degrees of accuracy in the plane of the silicon wafer. Control of tolerances in the third dimension is not as accurate. Vibrating structures for which the carrier and response mode motion is restricted to that of the wafer plane are therefore most appropriate to exploit the advantages offered by micromachining techniques.
Planar ring vibrating structures or resonators are an example of resonators which are particularly suited for fabrication using micromachining techniques. These are typically excited into a cos2.theta. resonance mode. For a perfectly symmetric resonator this mode actually exists as a degenerate pair of vibration modes at a mutual angle of 45.degree.. These are shown schematically in FIGS. 1A and 1B in which vibration of the structure is shown about primary axes P and secondary axes S. One of these modes is excited as the carrier mode as shown in FIG. 1A. When the structure is rotated about the axis normal to the plane of the ring (z-axis) Coriolis forces couple energy in to the response mode (FIG. 1B). The amplitude of motion of the response mode will be proportional to the applied rotation rate.
Vibrating structure gyroscopes employing planar ring resonators are described in GB 9703357.5 and U.S. Pat. No. 5,450,751. GB 9703357.5 describes a vibrating structure fabricated from bulk Silicon, incorporating a planar ring resonator externally supported by eight compliant support beams or legs. This gyroscope uses an inductive drive and pick-off method to generate the drive force to excite the resonator into motion and to sense the resultant motion. The gyroscope requires a magnetic circuit to be applied in the region of the ring structure. This design has a number of limitations. For example the magnetic circuit components must be fabricated using standard machining techniques. These are then subsequently assembled in precise alignment to the resonator. The accuracy to which these components can be machined and aligned does not match that achieved by the micromachining processes. The inclusion of these components limits the degree of miniaturization possible with the result that the gyroscope is not suitable for size reduction without significantly degrading the performance.
Additionally certain aspects of the performance of such a gyroscope will be critically dependent upon the characteristics of the magnetic circuit and the magnitude of the B-field. The most significant of these is the scale factor which exhibits a B.sup.2 dependence. The B-field will vary significantly over the operational temperature range causing a pronounced scale factor temperature dependence.
The gyroscope described in U.S. Pat. No. 5,450,751 incorporates an electroformed planar metal ring resonator which is driven into resonance electrostatically with the induced motion being capacitively sensed. The drive and pick-off transducers are formed between the outer circumferential edge of the ring and discrete plates positioned concentrically around the ring. This structure has been designed to minimize the natural in-plane frequency whilst maintaining it above any input vibration frequency band. An additional requirement is to maintain the out-of-plane natural frequencies above that of the in-plane. It is desirable to minimize the width of the ring to satisfy both of these requirements. The resultant resonator design uses a structure with a ring width equal to the width of the ring support legs. This gives a structure where the combined stiffness of the legs is high in comparison to that of the ring. This means that the resonant frequency of the structure is predominantly determined by the support legs and mechanical balancing procedures, such as the laser balancing process described in GBA2292609A, cannot be applied.
Balancing of carrier and response mode frequencies is achieved by applying a DC voltage to specific transducer sites. These act as electrostatic springs which differentially shift the mode frequencies. These balancing electrodes occupy locations which could otherwise be used for drive or pick-off sites to maximise the overall device head gain thus improving the noise performance. This balancing technique also requires the use of an additional feedback loop in the control electronics which will itself add noise to the system where large offset voltages are required to balance the modes.
There is thus a requirement for an improved angular rate sensor, preferably with scalefactor performance which is substantially temperature independent, with high drive and pick-off transducer gain, which is capable of being mechanically balanced and which may be produced in small size.